Antiskid brake systems enhance the ability of a vehicle to safely stop on a surface having an unexpected low coefficient of friction. Known systems monitor variations in the rotational speed of at least one wheel of a vehicle relative to the speed of the vehicle to detect a rapid reduction in the speed of the wheel that typically indicates that the brake is forcing the wheel to lock. An antilock system maximizes the frictional force between the wheel and the road surface by generating a control signal that reduces the force that the brake applies to the wheel, thus permitting the wheel to continue to reduce the velocity of the vehicle by allowing limited wheel rotation at a speed less than that corresponding to the velocity of the vehicle.
Other antilock systems that use velocity logic also compare a sensed speed voltage signal proportional to the rotational speed of a braked wheel to a predetermined variable reference voltage signal having a value that is less than that of the wheel speed voltage signal. Normally the established rate of decrease of the reference voltage with respect to time (reference voltage curve) is limited to a maximum rate corresponding to a predetermined rate of decay. If the actual speed of the wheel decreases at a greater rate than the maximum permitted rate of deceleration, the wheel speed voltage signal becomes less than the reference voltage signal and a control circuit, which detects the change in relative values between the wheel speed voltage and reference voltage signals, generates a command to decrease the force that the brake applies to the wheel.
Antilock systems typically must contain means for canceling the antilock signal that relieves brake force to permit restoration of full braking force to the wheel when the speed of the wheel begins to increase again after the brake is released. One canceling method is to calculate a second reference voltage that is greater than a wheel speed signal that corresponds to the velocity of the vehicle. The rate of increase of the wheel speed signal is then limited to a finite rate of increase corresponding to a predetermined rate of speed increase of the wheel. Care must be exercised in reapplying the brake force, however, because the proper amount of force to apply is determined by the instantaneous frictional force between the wheel and the road surface over which the wheel is moving. The amount of friction between the wheel and road can vary greatly; therefore, an antilock system often has means to adapt the amount of braking force applied to the wheel to the traction conditions between the wheel and surface.
Other systems contain examples of failsafe systems for disabling the antilock system and returning full control of the brake to the operator of the vehicle. The antilock system might have to be disabled if, for example, the system failed while releasing the braking force. These systems typically prevent the brake release system from operating when a failure is sensed, thereby returning control over the application of braking force to the operator of the vehicle. Indicator means inform the operator of the vehicle of a problem in the antilock system. The ultimate objective of any failsafe system is to leave the operator of the vehicle in no worse a position during a system failure than if the system had never been incorporated in the vehicle braking system.
Some systems compare the wheel speed signal with the reference signal by means of complex analog electronics. These analog means, however, are susceptible to temperature, environmental and electromagnetic interference. Control over the application of braking force, as well as control over the failsafe systems, is typically produced by complex analog computation systems that are susceptible to similar interference. The susceptibility of the control components to temperature interference requires use of various compensation means and the importance of having precise values for the components of the resulting circuit requires using expensive precision circuit components. These other systems show little, if any, concern with the harmful effects of stray voltages and radio frequency interference on the performance of the antilock system.
The disadvantages associated with analog controllers has motivated some to use a digital controller. The advantages of digital control are most fully realized by using a microprocessor to process a control algorithm. These advantages include the ability to more rapidly respond to actual conditions by using interrupt driven systems algorithms and greater accuracy in controlling the ultimate releasing and reapplying of the braking force. An antilock controller using a microprocessor to perform the necessary signal processing and control computations should make the antilock system less expensive to produce because a relatively inexpensive microprocessor may replace expensive analog circuitry.
An antilock system that uses using a microprocessor to execute various algorithms, however, requires a failsafe system that recognizes and responds to a greater number of potential system failures because the antilock system is more complex. The complexities involved in producing safe microprocessor controlled antilock systems has apparently slowed their use in mass produced vehicles. Moreover, the accuracy and response time of an antilock system is limited by various factors, such as mechanical oscillations of the vehicle and the runout (or eccentricity) of the wheel axle of the vehicle which adversely affect the accuracy of measurements the wheel speed, and electromagnetic interference which adversely effects the ability of a digital control system to correctly process the measured wheel speed signal. Many of these considerations have not heretofore been solved, or even recognized, in the prior art.